Robot apparatus, and load absorbing apparatus and method

ABSTRACT

To appropriately detect overload which may break a motor or deform a body and reduce the overload in the motor. The DC component of a load torque is derived from the sum of absolute values of a torque applied to a link connected to the output shaft of a motor and the generated torque of the motor, and it is determined that overload has been applied when the DC component exceeds a first threshold value for a prescribed period of time or longer. In addition, considering such a characteristic that the variation of energy applied to the output shaft of a motor is in proportion to a product of the torque and the angular velocity of the motor, the AC component of the load torque is detected based on the variation of energy, and it is predicted that overload will be applied when the AC component exceeds a second threshold value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under 35U.S.C. § 120 to co-pending U.S. application Ser. No. 10/731,145, filedDec. 10, 2003 and is also based upon and claims priority under 35 U.S.C.§ 119 from Japanese Patent Application Nos. 2002-367353, filed Dec. 18,2002 and 2003-367477, filed Oct. 28, 2003, the entire contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a robot apparatus, and a load absorbingapparatus and method for absorbing load applied to a motor, andspecifically, to a robot apparatus, and a load absorbing apparatus andmethod for motors which are used as actuators for joint motion of amulti-joint type robot.

For more detail, this invention relates to a robot apparatus, and a loadabsorbing apparatus and method for appropriately detecting andcontrolling overload which is applied to a motor and may break the motoror deform the body of the robot apparatus, and more specifically, to arobot apparatus, and a load absorbing apparatus and method, which areapplied to a multi-joint type robot comprising a plurality of actuatormotors in order to prevent breakage of members and the body due tooverload applied to the motors having a single or multiple axes.

2. Description of the Related Art

“Robot” is a mechanical apparatus which performs like human beings, withelectrical or magnetic functions. This term “robot” is said to beoriginated from “ROBOTA (slave machine)” in Slavic. In Japan, robotswere started to be spread in the late 1960s, and many of them wereindustrial robots such as manipulators and carrier robots for automatedand unmanned factories. However, recent development relating totwo-legged walking robot have been highly expected to be put topractical use. Specifically, the two-legged walking robot which ismodeled after human motion is called humanoid robot.

As compared with crawler, four-leg, or six-leg type robots, two-leg typerobot is unstable when moving, and its posture and walking are hard tobe controlled. However, the two-leg type robot is capable of waking onuneven floors and going up and down steps and ladders, which is anadvantage.

This kind of two-legged walking robot generally has a degree of freedomin many joints and these joints are moved by actuator motors. That is, amotor's output shaft is connected to one end of a link composing a partsuch as an arm or a leg, via a reduction gear, and the other end of thelink is connected to a motor for next-joint motion. In order to realizedesired performance and postures, servo control is performed based onthe rotational position and rotational amount of each motor.

In general, a servo motor is used for realizing a degree of freedom in ajoint of a robot, because it is easy to use, it is compact and has hightorque, and it is superior in response. Specifically, an AC servo motoris brushless and maintenance free, so that it can be applied to anautomated machine which is desired to work in unmanned working space,for example, to a joint actuator of a legged robot which walks freely.With a rotor comprising a permanent magnet and a stator consisting ofmulti-phase (for example, three-phase) coil windings, the rotor of theAC servo motor produces rotary torque with a sine wave flux distributionand a sine wave current.

Advanced two-legged walking robot autonomously walks and moves. Inaddition, this robot is capable of standing up from a lying position andholding and carrying objects with its arms. On the other hand, overloadmay be applied to its joint actuators when it falls down, bumps againstsomething, and gets something into its body.

Such overload may cause fatal damage, for example, breakage or plasticdeformation of its body. Therefore, what is crucial is that each motorconstituting a joint actuator is provided with a mechanism for absorbingload.

FIG. 1 shows a simple robot model. That is, the robot drives a motor 120under the control of a higher-ranked controller not shown, and givesoutput torque to a link 122 via a gear 121, so as to move a movablepart.

In this figure, a torque limiter is provided between the gear 121 andthe link 122, in order to absorb shocks to be given from the outside tothe link 122, thereby being capable of previously preventing breakage ofthe motor 120 and so on, caused by the shocks, such as deformation ofthe output shaft of the motor 120.

Various kinds of torque limiters (or servo savers) have been proposed(for example, refer to Japanese Patent Laid Open No. 60-192893). FIG. 2shows one example of the torque limiters.

In a torque limiter 130 of this figure, first and second semicircularfriction plates 132A and 132B are arranged inside a ring 131 fixed to alink 135. These first and second friction plates 132A and 132B are fixedto the output shaft 134 of a motor via elastic material 133 such asrubber or compression coil springs and are pressed against the ring 131by a fixed pressure caused by the elastic material 133.

In this torque limiter 130, the ring 131 can be generally rotatedtogether with the output shaft 134 of the motor by frictional forcegenerated between the first and second friction plates 132A and 132B andthe ring 131. However, when load greater than static friction forcebetween the first and second friction plates 132A and 132B and the ring131 is applied to the ring 131 due to a shock applied to the rink 135,the ring 131 and the first and second friction plates 132A and 132B slipon each other, so as not to cause load greater than kinetic frictionforce between the ring 131 and the first and second friction plates 132Aand 132B, in the output shaft 134 of the motor.

This conventional torque limiter 130, however, has a problem in thatstatic friction coefficients between the ring 131 and the first andsecond friction plates 132A and 132B vary easily and widely, so that itis difficult to determine an allowable margin at the time of designing arobot.

Further, in the conventional torque limiter 130, the static frictioncoefficients between the ring 131 and the first and second frictionplates 132A and 132B vary easily depending on temperature, which is alsoa problem.

Furthermore, since the conventional torque limiter 130 is constructedmechanically as described above, it is hard to construct a smaller andlighter torque limiter and to therefore realize a smaller and lighterrobot which contains motors.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of this invention is to provide anadvanced robot apparatus, and a load absorbing apparatus and method formotors which are used as actuators for joint motion of a multi-jointtype robot.

An additional object of this invention is to provide an advanced robotapparatus, and a load absorbing apparatus and method, which are capableof appropriately detecting overload which may break a motor and deformthe body, and reducing the overload in the motor.

An additional object of this invention is to provide an advanced robotapparatus, and load absorbing apparatus and method, which are capable ofappropriately preventing breakage of members and the body due tooverload applied to motors having a single or multiple axes, in amulti-joint type robot comprising a plurality of actuator motors.

As the first aspect, the foregoing objects and other objects of theinvention have been achieved by a robot apparatus having a plurality ofmovable joints, the robot apparatus comprising a plurality of motors fordriving the joints, a plurality of first overload detection means fordetecting overload in the corresponding motor, a load absorbing controlmeans for, when any of the first overload detection means detects theoverload in the corresponding motor, controlling a process to absorb theoverload in the motor, a second overload detection means for determiningwhether loads in two or more motors are totally excessive, and a bodyprotection control means for, when the second overload detection meansdetects the totally overload, carrying out a prescribed body protectionoperation.

Multi-joint type robot including two-legged walking robot is constructedby a plurality of motors for joint actuators and links connected to theoutput shafts of the motors. In this construction, it may happen thatloads in the plurality of motors are totally excessive even the load ineach motor is not excessive. For this case, it is considered that a bodyprotection operation for removing the overload state from the entirebody is required, separately from a load absorbing operation for eachmotor.

In the robot apparatus of the first aspect of this invention, the secondoverload detection means is provided so that it is detected whether anypart such as an arm or a leg, or the entire body is in an overload stateeven each motor is not in an overload state. When loads in a pluralityof motors are totally excessive, not the load absorbing operation foreach motor but the body protection operation is carried out.

The body protection operation here includes cutoff of power to relevantmotors or all motors of the body and weakening of the relevant motors orall motors of the body. The weakening of a motor is realized by settingits generated torque to zero or decreasing its viscosity resistance byservo gain adjustment.

Further, as a second aspect, a load absorbing apparatus for absorbingload applied to a motor comprises a torque measuring means for measuringa load torque based on the sum of absolute values of a torque applied toa link connected to the output shaft of the motor and the generatedtorque of the motor, an overload detection means for determining thatoverload has been applied when the load torque detected by the torquemeasuring means exceeds a first threshold value for a prescribed periodof time or longer, and a load absorbing control means for absorbing theoverload in the motor when the overload detection means detects theoverload.

Still further, as a third aspect of this invention, a load absorbingapparatus for absorbing load applied to a motor comprises a kineticenergy measuring means for measuring kinetic energy given to the outputshaft of the motor, an overload detection means for determining thatoverload will be applied when the variation of the kinetic energymeasured by the kinetic energy measuring means exceeds a secondthreshold value, and a load absorbing control means for avoiding theoverload in the motor when the overload detection means detects theoverload.

Still further, as a fourth aspect of this invention, a load absorbingapparatus for absorbing load applied to a motor comprises a torquemeasuring means for measuring a load torque based on the sum of absolutevalues of a torque applied to a link connected to the output shaft ofthe motor and a generated torque of the motor, a kinetic energyvariation measuring means for measuring the variation of kinetic energygiven to the output shaft of the motor, an overload detection means fordetecting overload based on the load torque measured by the torquemeasuring means and the variation of kinetic energy measured by thekinetic energy variation measuring means, and a load absorbing controlmeans for absorbing the overload in the motor when the overloaddetection means detects the overload.

Multi-joint type robot including two-legged walking robot is generallyconstructed by a plurality of motors for joint actuators, and linksconnected to the output shafts of the motors. Now, assume that overloadis applied to motors of the joint actuators when the robot falls down,dumps into something, or gets something into the body while walking ormoving. The overload may cause fatal damage such as breakage or plasticdeformation of the body (breakage of a link, dropout of a teeth of areduction gear).

Load torque to be applied to a motor for a joint actuator is broadlyclassified into impulsive “shock load” which incurs distortion energyand may break members such as links, due to bumping or the like and“constant load” which is relatively high load torque, although not sohigh as the shock load, and is constantly applied for a prescribedperiod of time or longer and thereby causes plastic deformation. Theinventors of this invention position the shock load and the constantload as AC component and DC component, respectively.

According to the second aspect of this invention, constant load, thatis, the DC component of the load torque is detected based on the sum ofabsolute values of the torque to be applied to a link connected to theoutput shaft of the motor and the generated torque of the motor, andthen it is recognized that the DC component of the load is excessivewhen the load torque exceeds the first threshold value for a prescribedperiod of time or longer. Since motor torque is in proportion to motorconducting current, the torque can be measured by converting motorcurrent into a voltage.

The first threshold value here is a value around the stall torque of themotor being used or a threshold value such as a limitation for circuitprotection.

Then, in order to avoid the breakage due to the constant load, aprescribed load absorbing operation can be carried out, for example, thegenerated torque of the motor is reduced or the viscosity coefficient ofthe motor is decreased. Such load absorbing apparatus realizes a smallvariation of motor viscosity coefficients among parts and further islightly affected by temperature. In addition, the torque detection canbe carried out by a simple process, which can realize a smaller andlighter robot apparatus even it contains motors.

Further, according to the third aspect of this invention, a load torqueis detected based on the variation of kinetic energy, considering suchcharacteristic that the variation of kinetic energy given to the outputshaft of a motor is in proportion to a product of a torque applied tothe motor and its angular velocity, and then it can be predicted thatthe AC component of the load will be excessive when the load torqueexceeds the second threshold value which may break members.

Then, in order to avoid breakage due to the shock load, a prescribedload absorbing operation can be carried out, for example, the generatedtorque of the motor is reduced or the viscosity coefficient of the motoris decreased. Such load absorbing apparatus has a small variation ofviscosity coefficients among parts and is lightly affected bytemperature. In addition, the torque is detected by a simple process,which can realize a smaller and lighter robot apparatus even it containsmotors.

Since the AC component of load torque is impulsive, overload is appliedat a moment and therefore the load absorbing operation may not be donein time. If based on the aforementioned characteristic in which thevariation of kinetic energy is in proportion to a torque applied to themotor and its angular velocity, the overload can not be detected untilthe torque reaches an overload state. This is a problem in response.

For this problem, such process can be effective that, not the variationof kinetic energy is measured simply, but the kinetic energy isdouble-differentiated with respect to time, with the result that it ispredicted that the AC component will become excessive when theinclination of the variation is a prescribed value or more, and then aload absorbing operation is performed.

In addition, the double differentiation can be approximated by a productof the rate of change of a torque and the angular velocity. On the otherhand, considering a characteristic in which motor torque is inproportion to motor current, the torque can be measured by convertingthe motor current into a voltage, so that the rate of change of thetorque can be measured via the time differentiation of the voltage.

Still further, in the load absorbing apparatus according to the fourthaspect of this invention, a load absorbing operation can be carried outbased on both the AC component and the DC component of a load torqueapplied to a motor, so that the two-legged walking robot can cope withvarious shocks received, without any expectation while autonomouslyperforming.

According to the load absorbing apparatus of the second to fourthaspects of this invention, in a case where overload is applied to thesingle axis of a motor, the AC component and the DC component of theload are properly absorbed, which can avoid breakage of the motor andmembers such as a link connected to the output shaft of the motor, andthereby preventing spreading of damage to other members.

As a result, this invention can provide an advanced robot apparatus, anda load absorbing apparatus and method for motors used as actuators forjoint motion of a multi-joint type robot.

Further, this invention can provide an advanced robot apparatus, and aload absorbing apparatus and method, which are capable of properlydetecting overload which may break motors or deform the body, andreducing the overload in the motors.

Still further, this invention can provide an advanced robot apparatus,and a load absorbing apparatus and method, which are capable ofappropriately preventing breakage of members and the body due tooverload applied to motors having a single or multiple axes in amulti-joint type robot comprising a plurality of actuator motors.

The nature, principle and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by like reference numerals or characters.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a conceptual drawing explaining motion of a movable part in aconventional robot;

FIG. 2 is a conceptual drawing schematically showing the construction ofa conventional torque limiter;

FIGS. 3 and 4 are perspective drawings showing an external constructionof a robot 1 according to this embodiment;

FIG. 5 is a schematic drawing explaining an external construction of therobot 1;

FIGS. 6 and 7 are block diagrams explaining an internal construction ofthe robot 1;

FIG. 8 is a schematic view showing a structure of software controlperformed in the robot 1;

FIG. 9 is a schematic view showing an internal structure of a middlewarelayer;

FIG. 10 is a schematic view showing an internal structure of anapplication layer;

FIG. 11 is a schematic view showing an internal structure of an actuatorA₁ to A₂₄;

FIGS. 12A to 12C are drawings showing the constructions of a rotor shaft64 and a rotor-shaft magnetic-pole angle sensor;

FIG. 13 is a drawing showing a positional relationship among a rotor 66and stator cores 67A to 67F;

FIGS. 14A to 14C are drawings showing the construction of a torqueamplification unit 61;

FIGS. 15A, 15B and 16 are drawings showing the construction of a controlsubstrate 77;

FIG. 17 is a drawing showing the constructions of a control IC 80 and adriving circuit 81;

FIG. 18 is a view showing excess response characteristics of coilcurrent in a case where the terminal ends of a coil are in an open stateand in a short state;

FIG. 19 is a view showing an example of a waveform of a BRAKE_(—)PWMcontrol signal;

FIG. 20 is a specific view showing the construction of an additionallogical circuit 92;

FIG. 21 is a view showing a relationship between a duty cycle of theBRAKE_(—)PWM control signal S14 and a viscosity coefficient of a motorunit 60;

FIG. 22 is a view showing AC component and DC component of a load torqueapplied to an actuator;

FIGS. 23 and 24 are drawings explaining a mechanism of absorbing loadaccording to this invention;

FIG. 25 is a flowchart showing a first load absorbing processingprocedure RT1 to be carried out by an arithmetic processing unit 90 ofthe control IC 80;

FIG. 26 is a flowchart showing a second load absorbing processingprocedure RT1 to be carried out by an arithmetic processing unit 90 ofthe control IC 80;

FIG. 27 is a view showing an example of a result of torque detection;

FIG. 28 is a drawing showing a situation in which the robot 1 putssomething between the legs in the middle of standing up from a lyingposition;

FIG. 29 is a flowchart showing a processing procedure to detect shockload and constant load in an actuator having a single axis, to detectoverload in an actuator motor having plural axes, and to perform a bodyprotection operation in response to detection of these loads;

FIG. 30 is a schematic view showing an operation on software to performa load absorbing operation and a recovery operation for a case whereshock load is applied to an actuator;

FIG. 31 is a schematic view showing an operation on software to performa load absorbing operation and a recovery operation for a case whereconstant load is applied to an actuator; and

FIG. 32 is a schematic view showing a structure of the middleware layerfor a case where load in a plurality of joint axes is determined to betotally excessive and a body protection operation is performed.

FIG. 33 is a schematic view showing an operation on software to performa load absorbing operation and a recovery operation for a case whereloads in a plurality of joint axes are totally excessive.

FIG. 34 is a graph of a response characteristic of the load absorbingmechanism.

DETAILED DESCRIPTION OF THE EMBODIMENT

Preferred embodiments of this invention will be described with referenceto the accompanying drawings:

(1) Construction of Robot of This Embodiment

FIG. 3 and FIG. 4 show an entire construction of a two-legged walkingrobot according to one embodiment of the present invention. In addition,FIG. 5 schematically shows a structure of a degree of freedom in therobot. Reference numeral 1 shows a two-leg walking type robot as awhole, in which a head unit 3 is placed on a body unit 2, arm units 4A,4B having the same construction are provided at upper left and rightparts of the body unit 2, respectively, and leg units 5A, 5B having thesame construction are provided at lower left and right parts of the bodyunit 2, respectively.

The body unit 2 is constructed of an upper body frame 10 and a waistbase 11 forming a lower body both of which are connected to each othervia a waist joint mechanism 12. By driving each of actuators A₁, A₂ ofthe waist joint mechanism 12 fixed to the waist base 11, the upper bodycan be rotated independently around a roll axis 13 and a pitch axis 14which are orthogonal to each other as shown in FIG. 5.

Further, the head unit 3 is fixed on the center upper part of a shoulderbase 15 fixed on the upper end of the frame 10 via a neck jointmechanism 16. By driving each of actuators A₃, A₄ of the neck jointmechanism 16, the head unit 3 can be rotated independently around apitch axis 17 and a yaw axis 18 which are orthogonal to one another asshown in FIG. 5.

Furthermore, the arm units 4A, 4B are fixed to the left and right of theshoulder base 15, respectively, via a shoulder joint mechanism 19. Bydriving actuators A₅, A₆, A₉, A₁₀ of the shoulder joint mechanism 19,the arm units 4A, 4B can be rotated independently around a pitch axis 20and a roll axis 21 which are orthogonal to one another as shown in FIG.5.

In this case, as to the arm units 4A, 4B, actuators A₈ and A₁₂ formingforearms are connected to the output shafts of actuators A₇ and A₁₁forming upper arms, respectively, via elbow joint mechanisms 22, and ahand unit 23 is attached to the distal end of each forearm. The forearmscan be rotated around a yaw axis 24 shown in FIG. 5 by driving theactuators A₇ and A₁₁. In addition, the forearms can be rotated around apitch axis 25 shown in FIG. 5 by driving the actuators A₈ and A₁₂.

On the other hand, each leg unit 5A, 5B is attached to the waist base 11via a hip joint mechanism 26. By driving actuators A₁₃–A₁₈ and A₁₉ toA₂₄ of the hip joint mechanism 26, the leg units 5A, 5B can be rotatedindependently around a yaw axis 27, a roll axis 28, and a pitch axis 29which are orthogonal to one another as shown in FIG. 5.

In this example shown, a lower-leg frame 32 is connected to the low endof a thigh frame 30 via a knee joint mechanism 31, and a foot unit 34 isconnected to the low end of the frame 32 via an ankle joint mechanism33. In each leg unit 5A, 5B, the lower leg can be rotated around a pitchaxis 35 shown in FIG. 5 by driving an actuator A₁₆ or A₂₂ of the kneejoint mechanism 31. In addition, the foot unit 34 can be rotatedindependently around a pitch axis 36 and a roll axis 37 which areorthogonal to each other as shown in FIG. 5, by driving actuators A₁₇,A₁₈, or A₂₃, A₂₄ of the ankle joint mechanism 33.

FIG. 6 schematically shows a control structure of the two-legged walkingrobot 1 according to this embodiment. On the back of the waist base 11,is arranged a control unit 42 housing a main control unit 40 forcontrolling the entire operation of the robot 1, peripheral circuitry 41including a power circuit and a communication circuit, a battery 45,etc.

FIG. 7 schematically shows an internal structure of the control unit 42.This control unit 42 is connected to sub-control units 43A–43D arrangedin respective constituent units (body unit 2, head unit 3, arm units 4A,4B, and leg units 5A, 5B), so as to supply necessary power voltages tothe sub-control units 43A to 43D and to communicate data with the units43A to 43D.

Connected to the actuators A₁–A₂₄ of the corresponding constituentunits, each sub-control unit 43A–43D is designed to be able to drivecorresponding actuators A₁–A₂₄ in a manner specified by various commandsgiven from the main control unit 40.

Furthermore, as shown in FIG. 7, at predetermined positions on the headunit 3 are arranged various external sensors such as a Charge CoupledDevice (CCD) camera 50 functioning as “eyes” of the robot 1 and amicrophone 51 as “ears”, and an output unit such as a loudspeaker 52 asa “mouth”. On each of the palms of the hand units 23 and the soles ofthe foot units 34 is arranged a touch sensor 53 as an external sensor.In addition, inside the control unit 42 is arranged various internalsensors including a battery sensor 54 and an acceleration sensor 55.

The CCD camera 50 captures surrounding environment and sends a capturedvideo signal S1A to the main control unit 40. The microphone 51 collectsexternal sounds and sends an obtained audio signal S1B to the maincontrol unit 40. In addition, the touch sensor 53 detects physicalpressures from a user and physical contacts with the outside, and sendsa detected result to the main control unit 40 as a pressure signal S1C.Furthermore, the battery sensor 54 periodically detects an energy levelof the battery 45 serving as the main power source, and sends thedetected result to the main control unit 40 as a battery level signalS2A. The acceleration sensor 56 periodically detects acceleration inthree axes (x axis, y axis, and z axis), and sends the detected resultsto the main control unit 40 as an acceleration signal S2B.

The main control unit 40 detects surrounding and internal conditions ofthe robot 1, contacts with an external entity, etc. based on the videosignal S1A, the audio signal S1B, the pressure signal S1C, etc., beingexternal sensor's outputs, and the battery level signal S2A, theacceleration signal S2B, etc. being internal sensor's outputs.

Then the main control unit 40 determines a subsequent action based onthe detected results, a control program being stored in an internalmemory 40A, and various control parameters being stored in an externalmemory 56 being installed, and sends control commands based on thedetermined results to relevant sub-control units 43A–43D. As a result,the specified actuators A₁–A₂₄ are set in motion based on the controlcommands and under the control of the sub-control units 43A–43D, thusletting the robot 1 take action, such as moving the head unit 3 up anddown, left to right, raising the arm units 4A, 4B, and walking.

In addition, the main control unit 40 recognizes user's conversationthrough an audio recognition process based on the audio signal S1B,gives the loudspeaker 52 an audio signal S3 for response, resulting inoutput of synthesized sounds for communication with the user.

As described above, the robot 1 is capable of behaving autonomouslybased on surrounding and internal conditions, and also capable ofcommunicating with the user.

FIG. 8 schematically shows a structure of control software operated on atwo-legged walking robot 1 according to this embodiment.

As shown in this figure, the robot control software is multilayersoftware in which an object-oriented programming is adopted. Eachsoftware comprises modules called “objects” integrating data andprocessing of the data.

A device driver of the lowest layer comprises objects which are allowedto directly access hardware for driving of joint actuators, reception ofsensor outputs and so on, and performs a suitable process in response toan interrupt request from the hardware.

A virtual robot is an object which mediates between various devicedrivers and an object operating based on inter-object communicationprotocol. Each of hardware units composing the leg-type walking robot 1is accessed via this virtual robot.

A service manager is a system object which induces each object to make aconnection, based on information on connection between the objectsstored in a connection file.

Since each software having a higher ranking than a system layer (OS)comprises modules each representing an object (process), the objects areselectable and replaceable according to necessary functions. Byrewriting the connection file, the inputs and outputs of objects havingthe same data structure are connected as desired.

Software modules other than the device driver layer and the system layerare broadly classified into a middleware layer and an application layer.

FIG. 9 schematically shows an internal structure of the middlewarelayer.

The middleware layer is a collection of software modules to providebasic functions of the two-legged walking robot 1, the structure of eachmodule is affected by hardware attribution such as mechanical andelectrical features, specifications, and shapes of the two-leggedwalking robot 1. This middleware layer is functionally divided into arecognition middleware (the left half in FIG. 9) and an outputmiddleware (the right half in FIG. 9).

The recognition middleware receives and processes raw data from thehardware via the virtual robot, the raw data including video data, audiodata and other data obtained from other sensors. For example, thismiddleware performs various processes such as audio recognition,distance detection, posture detection, contact detection, motiondetection, and color recognition, based various input information, torecognize the robot's situation (for example, “detected a ball”, “felldown”, “being pat”, “being hit”, “heard musical scales”, or “detected amoving entity or an obstacle”). This recognized situation is notified tothe higher-ranked application layer via an input semantics converter tobe used for a subsequent action plan or learning.

The output middleware, on the other hand, realizes functions forwalking, moving, output of synchronized sounds, blinking control of theLEDs functioning as eyes. That is, the output middleware receives theaction plan made by the application layer via an output semanticsconverter, and creates a servo command value for each joint of thetwo-legged walking robot 1, and sounds and light (LEDs) to be output, inorder to let the robot 1 perform based on the plan via the virtualrobot. In addition, this output middleware of this embodiment includes aload monitoring module for detecting whether load in a plurality ofactuator motors are totally excessive and a module for executing a bodyprotection operation when an overload state is detected (this will bedescribed later).

FIG. 10 schematically shows an internal structure of the applicationlayer. This application layer is composed of one or more applicationsoftwares for controlling robot's performance and internal conditionssuch as instinct and emotions.

This application uses the recognized situation received via the inputsemantics converter to determine the action plan of the lag-type movingrobot 1, and notifies the determined plan via the output semanticsconverter.

The application is composed of an emotional model, an instinct model, alearning module for sequentially storing experienced events, abehavioral model storing behavior patterns, and an action changing unitfor moving the robot 1 based on the action plan determined by thebehavioral model.

The recognized situation input via the input semantics converter isinput to the emotional model, the instinct model, and the behavioralmodel, and also to the learning module as a learning instruction signal.

The action plan for the two-legged walking robot 1 determined by thebehavioral model is sent to the middleware via the action changing unitand the output semantics converter, so that the robot 1 takes theaction. This action plan is also given to the emotional model, theinstinct model and the learning module via the action changing unit asan action history.

Based on the recognized situation and the action history, the emotionalmodel and the instinct model control the emotional parameter andinstinct parameter, respectively. The behavioral model can refer tothese parameters. In addition, the learning module updates anaction-selection probability based on the learning instruction signal,and supplies the updated contents to the behavioral model.

(2) Construction of Actuators A₁ to A₂₄

As described above, a degree of freedom in each joint of the two-leggedwalking robot 1 according to this embodiment is realized by theactuators A₁ to A₂₄.

FIG. 11 shows the internal construction of each actuator A₁ to A₂₄. Asshown in this figure, each actuator A₁ to A₂₄ is composed of a motorunit 60 for generating a rotational torque and a torque amplificationunit 61 for amplifying and outputting the rotational torque.

In the motor unit 60, a rotor shaft 64 rotatably supported by bearings63A, 63B is provided inside a motor case 62 made of conducting materialsuch as metal. FIGS. 12A to 12C show the constructions of the rotorshaft 64 and a rotor-shaft magnetic-pole angle sensor. As shown in FIGS.12A and 12B, a rotor 66 is formed in such a manner that a rotor base 65and a rotor magnet 95 which is a double pole ring permanent magnet areintegrated coaxially with the rotor shaft 64.

FIG. 13 shows a relationship among the rotor 66 and stator cores 67A to67F. As shown in this figure, inside the motor case 62, six stator cores67A to 67F are fixed every 60 degrees around the rotor 66. Three phasecoils 68 (68A, 68B, 68C) are formed by winding a wire on the statorcores 67 (67A to 67F).

As shown in FIG. 13, three pairs of two opposite coils 63 are U-, V-,W-phase. By applying coil currents having 120-deegree phase differenceto the U-, V-, W-phase coils 68, each coil 68 generates a magnetic fieldcorresponding to the drive current, so that the rotor 66 can generate arotational torque corresponding to the coil current.

FIGS. 14A to 14C show the construction of the torque amplification unit61. As apparent from FIG. 11, the torque amplification unit 61 has agear case 69 detachably fixed to one end of the motor case 62. Insidethis gear case 69, a planetary gear mechanism 73 is provided, theplanetary gear mechanism 73 having a ring internal gear 70 attached tothe inside of the gear case 69, a sun gear 71 fixed to an end of therotor shaft 64, and first to third planetary gears 72A to 72C arrangedevery 120 degrees between the internal gear 70 and the sun gear 71.

In addition, each shaft 74A to 74C of the first to third planetary gears72A to 72C are fixed to the output shaft 75 rotatably arranged at an endof the gear case 69. Therefore, the torque amplification unit 61 canamplify a rotational torque given from the motor unit 60 via the rotorshaft 64, via the planetary gear mechanism 73 and then output theresultant to the outside via the output shaft 75.

In addition, the motor case 62 of the motor unit 60 contains an encoder76 for detecting rotations of the rotor shaft 64 and a control substrate77 for controlling a rotation angle and a rotational torque of the rotorshaft 64 in response to an operation command from a higher-rankedcontroller (corresponding sub-control unit 43A to 43D) (see FIG. 11).

In this case, the encoder 76 is composed of the double-pole resin magnet78 and first and second hole elements 79A, 79B (see FIGS. 12A to 12C).FIGS. 15A and 15B show the construction of the control substrate 77. Asshown in FIG. 15B, the first and second hole elements 79A, 79B arearranged coaxially with the rotor shaft 64 at 90 degree phase differenceon the control substrate 77.

Therefore, the encoder 76 is capable of detecting the rotationalposition of the rotor shaft 64 as the change of magnetic flux density atthe first and second hole elements 79A, 79B, the change going with therotation of the resin magnet 78 which rotates together with the rotorshaft 64.

Further, as shown in FIG. 16, in the control substrate 77, a control IC80 and a drive circuit 81 for supplying drive currents Iu, Iv, Iw to thecoils 68 u, 68 v, 68 w of the motor unit 60 under the control of thecontrol IC 80 are arranged on a circular printed wiring board. Thecontrol IC 80 is capable of receiving the outputs of the first andsecond hole elements 79A, 79B as first and second position signals S10A,S10B, respectively, and thereby detecting the rotational position of therotor shaft 64 based on the signals.

The control substrate 77 is connected to a higher-ranked controller(corresponding sub control unit) via a cable 83 (see FIG. 11), so thatthe control IC 80 can communicate with the higher-ranked controller viathe cable 83 and be supplied with power voltage Vcc.

Then, the control IC 80 controls the drive circuit 81 based on theoperation command COM given from the higher-ranked controller via thecable 83 and the first and second position signals S10A, S10B, to applythe drive currents Iu, Iv, Iw to corresponding U-, V-, and W-phase coils68 of the motor unit 60, so that the motor unit 60 can rotate by arotation angle or generate a rotational torque depending on theoperation command COM.

(3) Constructions of Control IC 80 and Drive Circuit 81

FIG. 17 shows the constructions of the control IC 80 and the drivecircuit 81. As shown in this figure, the control IC 80 is composed of anarithmetic processing unit 90, a PWM control unit 91, and an additionallogical circuit 92.

The arithmetic processing unit 90 is constructed like a microcomputerincluding a CPU, ROM and RAM, and this unit 90 obtains a differencebetween a targeted rotational position of the rotor shaft 64 of themotor unit 60 based on the operation command COM from the higher-rankedcontroller and the current rotational position of the rotor shaft 64obtained based on the first and second position signals S10A, SLOB givenfrom the encoder 76, and also calculates a targeted output torque(hereinafter, referred to as a targeted torque) in order to eliminatethe difference, and sends the calculation result to the PWM control unit91 as a torque command signal S12.

The PWM control unit 91 PWM-controls the drive circuit 81 via theadditional logical circuit 92 based on the torque command signal S12 sothat the motor unit 60 can generate the targeted torque as its outputtorque.

The drive circuit 81 is composed of a pair of PNP transistor TRu1 andNPN transistor TRu2 for U phase, a pair of PNP transistor TRv1 and NPNtransistor TRv2 for V phase, and a pair of PNP transistor TRw1 and NPNtransistor TRw2 for W phase.

Each emitter of the PNP transistors TRu1, TRv1, TRw1 is connected to thepower voltage Vcc, the collectors of the PNP transistors TRu1, TRv1,TRw1 are connected to the collectors of the NPN transistors TRu2, TRv2,TRw2, respectively, and each emitter of the NPN transistors TRu1, TRv1,TRw1 is grounded. In addition, the connecting points P1 to P3 connectingthe collectors of the PNP transistors TRu1, TRv1, TRw1 and thecollectors of the corresponding NPN transistors TRu2, TRv2, TRw2 areconnected to a connecting point P4 between the U-phase coil 68 u and theW-phase coil 68 w, a connecting point P5 between the U-phase coil 68 uand the V-phase coil 68 v, and a connecting point P6 between the V-phasecoil 68 v and the W-phase coil 68 w.

By turning ON the PNP transistor TRu1 and the NPN transistor TRv2 andturning OFF the NPN transistor TRu2 and the PNP transistor TRv1, theU-phase coil 68 u becomes conductive to flow the drive current IU in anarrow direction. In addition, by turning OFF the PNP transistors TRu1,TRv1 and the NPN transistors TRu2, TRv2, the terminals P4, P5 are put inan open state and the U-phase coil 68 u becomes nonconductive.

Similarly, by turning ON the PNP transistors TRv1 and the NPN transistorTRw2 and turning OFF the NPN transistor TRv2 and the PNP transistorTRw1, the V-phase coil 68 v becomes conductive to flow the drive currentIV in an arrow direction. In addition, by turning OFF the PNPtransistors TRv1, TRw1 and the NPN transistors TRv2, TRw2, the terminalsP5, P6 are put in an open state and the V-phase coil 68 v becomesnonconductive.

Further, similarly, by turning ON the PNP transistors TRw1 and the NPNtransistor TRu2 and turning OFF the NPN transistor TRw2 and the PNPtransistor TRu1 OFF, the W-phase coil 68 w becomes conductive to flowthe drive current IW in an arrow direction. In addition, by turning OFFthe PNP transistors TRw1, TRu1 and the NPN transistors TRw2, TRu2, theterminals P6, P4 are put in an open state and the w-phase coil 68 wbecomes nonconductive.

As a result, the PWM control unit 91 gives first to sixth PWM signalsS13 u 1, S13 u 2, S13 v 1, S13 v 2, S13 w 1, S13 w 2 corresponding tothe targeted torque obtained based on the torque command signal S12, tothe bases of the corresponding PNP transistors TRu1, TRv1, TRw1 and theNPN transistors TRu2, TRv2, TRw2, to thereby switch each of the U-, V-,and W-phase coils 68 u, 68 v, 68 w between the conductive state and thenonconductive state, resulting in rotation of the motor unit 60.

Note that, in the case where the position of the motor unit 60 iscontrolled by the PWM control, putting the terminals P4, P5, P6 in theopen state while the coils 68 u, 68 v, 68 w are nonconductive arisesproblems in that a torque loss occurs because current existing in themotor unit 60 (strictly, charge) is easy to be lost and cogging torqueis easy to affect the control.

However, this embodiment solves the torque loss problem by utilizingsuch a fact that currents (strictly charge) existing in the coils 68 u,68 v, 68 w do not decrease easily by putting the terminals P4, P5, P6 ofthe coils 68 u, 68 v, 68 w in the short state, not in the open state,while the coils 68 u, 68 v, 68 w are nonconductive. FIG. 18 shows theexcessive response characteristics of coil current for cases where theends of a coil are put in the open state and the short state. As clearfrom this figure, the short state requires a longer settling time, sothat the coil current does not decrease easily.

This is because putting the terminals P4, P5, P6 in the short statecauses a time delay in the excessive response which is resulted from DCresistance and inductance of the coils 68 u, 68 v, 68 w of the motorunit 60. In addition, in the short state, a magnetic flux density fromthe rotor magnet 95 generates a reverse electromotive force to the coils68 u, 68 v, 68 w. This reverse electromotive force is applied in areverse direction of rotation of the rotor 66 to generate a viscosityresistance against the rotation by an external force, which obtains aneffect like a break. Such viscosity resistance generated by creating aviscosity coefficient against the motor unit 60 can reduce the effect ofthe cogging torque without torque loss.

However, putting the terminals P4, P5, P6 of the coils 68 u, 68 v, 68 win the short state means applying a kind of viscosity resistance to themotor unit 60, which arises another problem in that a break due to theshort states of the terminals P4, P5, P6 of the coils 68 u, 68 v, 68 wloses compliance in the motor unit 60.

In this embodiment, to solve the above torque loss problem and thiscompliance problem together, the terminals P4, P5, P6 of the coils 68 u,68 v, 68 w are alternatively switched between the open state and theshort state while the coils 68 u, 68 v, 68 w are nonconductive, and aratio of a period for the open state to a period for the short state isadjusted so as to obtain desired compliance.

Actually, as a means for alternatively switching the terminals P4, P5,P6 of the coils 68 u, 68 v, 68 w between the open state and the shortstate, the additional logical circuit 92 is provided at a latter stageof the PWM control unit 91. The additional logical circuit 92 is given,for example, a BRAKE_(—)PWM control signal S14 of a short waveformtogether with the first to sixth PWM signals S13 u 1, S13 u 2, S13 v 1,S13 v 2, S13 w 1, S13 w 2, from the PWM control unit 91, the signal S14being subjected to PWM modulation according to a prescribed ratio of aperiod for the open state to a period for the short state regarding theterminal P4, P5, P6. FIG. 19 shows a waveform example of theBRAKE_(—)PWM control signal.

The additional logical circuit 92 changes, according to necessity, thelogical levels of the corresponding first to sixth PWM signals S13 u 1,S13 u 2, S13 v 1, S13 v 2, S13 w 1, S13 w 2 so as to put the terminalsP4, P5, P6 of the coils 68 u, 68 v, 68 w in the short state while thecoils 68 u, 68 v, 68 w are nonconductive and the BRAKE_(—)PWM controlsignal S14 has a logical level “1”.

Specifically, to put the terminals P4, P5 of the U-phase coil 68 u inthe open state while the coils are nonconductive, the PWM control unit91 normally outputs the first to fourth PWM signals S13 u 1, S13 u 2,S13 v 1, S13 v 2 to turn OFF the PNP transistors TRu1, TRuv1 and the NPNtransistors TRu2, TRv2. However, the additional logical circuit 92intermittently puts the terminals P4, P5 in the short state by turningON the PNP transistors TRu1, TRv1, which are currently off, while theBRAKE_(—)PWM control signal S14 has a logical level “1”.

Similarly, to put the terminals P5, P6 of the V-phase coil 68 v in theopen state while the coils are nonconductive, the PWM control unit 91normally outputs the third to sixth PWM signals S13 v 1, S13 v 2, S13 w1, S13 w 2 to turn OFF the PNP transistors TRv1, TRw1 and the NPNtransistors TRv2, TRw2. However, the additional logical circuit 92intermittently puts the terminals P5, P6 in the short state by turningON the PNP transistors TRv1, TRw1, which are currently OFF, while theBRAKE_(—)PWM control signal S14 has a logical level “1”.

Further, similarly, to put the terminals P6, P4 of the W-phase coil 68 win the open state while the coils are nonconductive, the PWM controlunit 91 normally outputs the first, second, fifth, and sixth PWM signalsS13 u 1, S13 u 2, S13 w 1, S13 w 2 to turn OFF the PNP transistors TRw1,TRu1 and the NPN transistors TRw2, TRu2. However, the additional logicalcircuit 92 intermittently puts the terminal P6, P4 in a short state byturning ON the PNP transistors TRw1, TRu1, which are currently OFF,while the BRAKE_(—)PWM control signal S14 has a logical level “1”.

In a case where the BRAKE_(—)PWM control signal S14 has a logical level“0” while the coils are nonconductive, on the other hand, the additionallogical circuit 92 normally outputs the first to sixth PWM signals S13 u1, S13 u 2, S13 v 1, S13 v 2, S13 w 1, S13 w 2 from the PWM control unit91 as they are, resulting in leaving the terminals P4, P5, P6 of thecoils 68 u, 68 v, 68 w in the open state.

FIG. 20 shows the specific construction of the additional logicalcircuit 92. An AND gate 100 performs AND operation on the first, thirdand fifth PWM signals S13 u 1, S13 v 1, and S13 w 1 from the PWM controlunit 91 while an exclusive NOR gate 101 performs exclusive NOR operationon the second, fourth, and sixth PWM signals S13 u 2, S13 v 2, S13 w 2.

Then, a NAND gate 102B performs NAND operation on the output of the ANDgate 100 and the output of the exclusive NOR gate 101, and an OR gate103 performs OR operation on the output of the NAND gate 102B and theoutput of an inverting buffer 102A, i.e., the signal opposite theBRAKE_(—)PWM control signal S14.

Then, first to sixth AND gates 104A to 104F perform AND operation on theoutput of the OR gate 103 and the original first to sixth PWM signalsS13 u 1, S13 u 2; S13 v 1, S13 v 2, S13 w 1, S13 w 2, respectively, andthe outputs of these first to sixth AND gates 104A to 104F are given tothe PNP transistor TRu1, NPN transistor TRu2, PNP transistor TRv1, NPNtransistor TRv2, PNP transistor TRw1, and NPN transistor TRw2,respectively.

As described with reference to FIG. 18, by putting the terminals P4, P5,P6 of the coils 68 u, 68 v, 68 w in the short state while the coils arenonconductive, the drive currents Iu, Iv, Iw of the coils 68 u, 68 v, 68w take a longer time to become zero because of the excessive response.Therefore, while the coils 68 u, 68 v, 68 w are nonconductive, currentstarts to flow before the drive currents Iu, Iv, Iw become zero, byrepeatedly switching the open state and the short state.

Therefore, the maximum values of the drive currents Iu, Iv, Iw to besupplied to the coils 68 u, 68 v, 68 w gradually increase every timewhen the coils 68 u, 68 v, 68 w become conductive and nonconductive, andthe increasing rate almost corresponds to a duty cycle, that is, a ratein which the BRAKE_(—)PWM control signal S14 becomes a logical level“1”. Similarly, the effective values of the drive currents Iu, Iv, Iw ofthe coils 68 u, 68 v, 68 w also gradually increase and the increasingrate almost corresponds to the duty cycle, that is, a rate in which theBRAKE_(—)PWM control signal S14 becomes a logical level “1”.

In addition, since the output torque of the motor unit 60 is calculatedby multiply a torque constant Kt of the motor unit 60 and a sum of thevalues of the drive currents Iu, Iv, Iw to be supplied to the coils 68u, 68 v, 68 w. Therefore, when the coils 68 u, 68 v, 68 w are repeatedlyswitched between the conductive state and the nonconductive state, theeffective value of the output torque in the motor unit 60 increases withincreasing the sum of the values of the drive currents Iu, Iv, Iw.

The increasing rate of this case almost corresponds to the duty cycle ofthe BRAKE_(—)PWM control signal S14, that is, a rate in which theBRAKE_(—)PWM control signal S14 becomes a logical level “1”. And theinclination of increase of the output torque of the motor unit 60 is inproportion to the viscosity coefficient of the motor unit 60. In otherwords, the viscosity coefficient of the motor unit 60 can be dynamicallyand optionally determined within a range of duty resolution with theduty cycle of the BRAKE_(—)PWM control signal S14.

As a result, the viscosity coefficient of the motor unit 60 can becontrolled by changing the duty cycle of the BRAKE_(—)PWM control signalS14, and the currents (the amount of charge) to flow in the coils 68 u,68 v, 68 w can be controlled while the coils 68 u, 68 v, 68 w arenonconductive.

When the duty cycle of the BRAKE_(—)PWM control signal S14 is determinedso that the viscosity coefficient becomes large, the motor unit 60 hashigh holding power, resulting in reducing disturbance such as coggingtorque. In addition, the amount of compliance toward an external forcecan be controlled.

The PWM control of the duty cycle of the BRAKE_(—)PWM control signal S14to be supplied to the additional logical circuit 92, which is performedby the PWM control unit 91, adjusts a ratio of the period for the openstate to the period for the short state in the terminals P4, P5, P6 ofthe coils 68 u, 68 v, 68 w while the coils are nonconductive, therebyobtaining desired compliance. FIG. 21 shows a relationship between theduty cycle of the BRAKE_(—)PWM control signal S14 and the viscositycoefficient of the motor unit 60.

(4) Load Absorbing Mechanism in Each Actuator A₁ to A₂₄

An advanced two-legged walking robot autonomously walks and moves.Further, the robot can make motion such as standing up from a lyingstate and holding and carrying objects with its arms. On the other hand,such case may arise that overload is applied to joint actuators due tofalling down, dumping into something, or getting something into itsbody. Such overload may cause fatal damage such as breakage or plasticdeformation of the body. Therefore, it is very important to provide eachmotor composing the joint actuators with a mechanism to absorb load.

First, a load absorbing function which is provided in each actuator A₁to A₂₄ for joint motion in this robot 1 will be described.

The inventors of this invention treat load torque to be applied to amotor for a joint actuator by broadly classifying it into impulsive“shock load” which incurs distortion energy and may break members suchas links, due to bumping or the like and “constant load” which isrelatively high load torque, although not so high as the shock load, andcauses plastic deformation (see FIG. 22) if constantly applied for aprescribed period of time or longer. The latter constant load torquevalue is a value around a stall torque of a motor used, or a thresholdvalue such as a limitation for circuit protection. The inventorsposition the shock load and the constant load as AC component and DCcomponent, respectively.

The constant load, that is, the DC component of a load torque isdetected based on the sum of absolute values of a load torque to beapplied to a link connected to the output shaft of an actuator motor anda generated torque by the actuator motor. Then, it is recognized thatthe DC component is excessive in a case where the total torque exceeds athreshold value which may cause plastic deformation when applied for aprescribed period of time or longer. Since the generated torque of amotor is in proportion to motor current, the torque can be measured byconverting the motor current into a voltage.

For example, as shown in FIG. 23, suppose that, in a situation where alink 112 of which one end is linked with the output shaft 111 of anactuator 110 can not move because it's the other end (which is L1distant from the output shaft 111) is contacting with a wall or anentity, the other end of the link 112 is given an external force F1 fromthe wall or the entity and the actuator 10 generates a torque by thenewly increased load.

In this case, a condition where the actuator 110 is not broken is asfollows:F 1×L1+T _(OUT) ≦T _(BRK)  (1)where T_(out) is the generated torque being generated firstly by theactuator 110 and T_(BRK) is a load torque which will break the actuator110 if applied to the output shaft 111 of the actuator 110. That is, thecondition is that the sum of absolute values of a load torque newlycaused by an external force F1 and the generated torque T_(out) by theactuator 110 does not exceed T_(BRK) for a prescribed period of time orlonger.

Such generated torque T_(out) corresponds to the DC component of loadand is referred to as “static load torque” hereinafter. Based on theabove equation (1), by controlling the generated torque T_(out) of theactuator 110 so as to satisfy the following equation, the actuator 110can be previously prevented from being broken.T _(OUT) ≦T _(BRK) −F 1×L1  (2)

The generated torque T_(out) of the actuator 110 can be detected as acoil current supplied to the actuator 110. Now, FIG. 27 shows currentvalues corresponding to torque, where a threshold value for the coilcurrent into which a threshold value for a load torque is converted is2.5A and a time limit for continuous of this threshold value is 2.0seconds. Continuous sampling of current values have a chatteringproblem. That is, even a value greater than 2.5A actually continues for2.0 seconds, a counter is reset because of the chattering every timewhen the value falls below the threshold value, which may miss detectiontiming. For such a case, such operation can be effective that, if aperiod of time when the value is below the threshold value is very short(for example, for only 4.0 milliseconds or shorter), the counter keepson counting.

As to the shock load, that is, the AC component of load, on the otherhand, the inventors introduced a fact in that the variation of kineticenergy to be applied to the output shaft of a motor is in proportion toa product of a torque to be applied to the motor and the angularvelocity. In addition, since the kinetic energy is almost equal todistortion energy added to the output shaft member, a load torquecorresponding to a shock is detected based on the variation of thekinetic energy, and the AC component of load is recognized excessivewhen the load torque exceeds a threshold value which may break a member.

For example, a case shown in FIG. 24 will be considered where such aload torque as to rotate the link 112 is applied to the other end of thelink 112 (which is L2 distance from the output shaft 111) due to, forexample, an entity falling on the other end. Such a load torquecorresponds to the AC component of load and is referred to as “dynamicload torque” hereinafter.

Kinetic energy K_(E) of this time, which is applied to the link 112 bythe dynamic load torque, is derived from the follows: $\begin{matrix}{K_{E} = {\frac{1}{2} \times J \times {\overset{.}{\phi}}^{2}}} & (3)\end{matrix}$where φ is a rotation angle of the link 112 and J is an inertia momentof the link 112.

Assuming that all of this kinetic energy K_(E) travel to the actuator110 through the output shaft 111 and are converted into distortionenergy. This distortion energy does not break the actuator 110 when itis under an elastic limit in the actuator 110; and the distortion energybreaks the actuator 110, otherwise.

When the distortion energy becomes over the elastic limit, the generatedtorque T_(out) of the actuator 110 is reduced in order to prevent thebreakage of the actuator 110 due to the dynamic load torque.

Specifically, to detect the variation of the kinetic energy K_(E) of thedynamic load torque applied to the link 112, the equation (3) issingle-differentiated with respect to time as follows: $\begin{matrix}{{\frac{\mathbb{d}}{\mathbb{d}t}( K_{E} )} = {{I \times \overset{¨}{\phi} \times \overset{.}{\phi}} = {\tau \times \overset{.}{\phi}}}} & (4)\end{matrix}$where τ represents an output torque of the actuator 110. That is, sincethe variation of energy is in proportion to a product of the torqueapplied to the motor and the angular velocity, the load torque isdetected based on the variation of energy, and the AC component of loadis recognized excessive when the load torque exceeds a threshold valuethat may break a member.

Now, since the AC component of a load torque is impulsive (refer to FIG.4), the component reaches an overload state at moment and therefore, aload absorbing operation may not be carried out in time. As describedabove, if based on a characteristic in which the variation of energy isin proportion to a product of a torque applied to a motor and theangular velocity, overload can not be detected until the torque reachesan overload state, which arises a problem in response.

Therefore, the variation of energy is not measured simply, but theinclination of the variation is considered by double differentiation ofthe energy with respect to time. Then, it is predicted that the torquewill reach an overload state if the inclination is more than aprescribed value, with the result that the load absorbing operation canbe carried out with good response. The equation (4) is furtherdifferentiated with respect to time as follows (that is, theaforementioned equation (3) is double-differentiated with respect timet): $\begin{matrix}{{\frac{\mathbb{d}^{2}}{\mathbb{d}t^{2}}( K_{E} )} = {{\overset{.}{\tau} \times \overset{.}{\phi}} + {\tau \times \overset{¨}{\phi}}}} & (5)\end{matrix}$

The equation (5) represents acceleration of variation of the kineticenergy K_(E). When the right-hand side of the equation (5) is aprescribed threshold value or greater which is a boarder between theelastic range and the plastic range of each member in the actuator 110,the generated torque T_(out) of the actuator 110 is reduced in order topreviously prevent breakage of the actuator 110.

However, the double differentiation process like the equation (5) is notsimple. In addition, the inventors realized that the second item of theright-hand side in the equation (5) makes a small contribution to theoperation result. Therefore, the second item is omitted and theright-hand side of the equation (5) is made approximate to the productof the first item of the right-hand side and a prescribed gain Gk, asfollows:f(t)=G _(k)×{dot over (τ)}×{dot over (φ)}  6)where f(t) is an evaluation function.

The generated torque T_(out) is reduced when the evaluation functionf(t) becomes over the above-described threshold value. As a result, theactuator 110 can be previously prevented from being broken, with thesimpler arithmetic process. FIG. 34 shows a comparison in a responsecharacteristic of the load absorbing mechanism between a case based onthe variation of kinetic energy K_(E) and a case in which the evaluationfunction f(t) is introduced. As clear from this figure, the latter casecan detect an overload state with a better response at a time of a lowergenerated torque.

The control substrate 77 operates as a load absorbing means forpreventing breakage of the actuators A₁ to A₂₄ due to the static ordynamic load torque, which was described with reference to FIG. 23 andFIG. 24, based on the aforementioned principles. As shown in FIG. 16, avoltage detection unit 82 for detecting as voltage V_(i) the amount of acurrent I_(R1) flowing in a power line LIN for the drive circuit 81 isprovided on the printed wiring board of the control substrate 77.

The voltage detection unit 82 is formed of a first resistance R₁provided on the power line LIN and a differential amplifier 84 composedof second to fifth resistances R₂ to R₅ and an operational amplifier 83.The differential amplifier 84 detects a fall voltage V_(i) in the firstresistance R₁ and sends the detected result to the control IC 80 as avoltage signal S11.

Since the voltage V_(i) is in proportion to the current I_(R1) flowingin the first resistance R₁ and this current I_(R1) is in proportion tothe left-hand side of the equation (1), the voltage V_(i) is also inproportion to the left-hand side of the equation (1). Now, it issupposed that the following equation is realized:F 1×L1+T _(out) =K _(vi) ×V _(i) ≦T _(BRK)  (7)where K_(vi) is a proportionality constant.

Therefore, the actuator A₁ to A₂₄ can be prevented from being broken dueto a static load torque by controlling the PWM control unit 91 so as tosatisfy the following equation: $\begin{matrix}{V_{i} \leqq \frac{T_{NRK}}{K_{vi}}} & (8)\end{matrix}$

FIG. 25 is a flowchart of the first load absorbing processing procedureRT1 which is performed by the arithmetic processing unit 90 of thecontrol IC 80. The arithmetic processing unit 90 controls the PWMcontrol unit 91 based on the voltage signal S11 given from the voltagedetection unit 82 so as that the voltage V_(i) does not exceed aprescribed preset first threshold value SH1 (T_(BRK)/K_(vi) in theequation (8)), which prevents the actuator A₁ to A₂₄ from being brokenin a case where the static load torque, that is, the DC component of theload is applied to the corresponding link.

The arithmetic processing unit 90 starts the first shock absorbingprocessing procedure RT1 from step SP0, in parallel to the positionalcontrol of the corresponding motor unit 60 based on the operationcommand COM given from the higher-ranked controller as described above,and judges at step SP1 whether the voltage V_(i) obtained based on thevoltage signal S11 given from the voltage detection unit 82 is the firstthreshold value SH1 or greater. When a negative result is obtained atstep SP1, the arithmetic processing unit 90 repeats this step SP1 untilan affirmative result is obtained.

Then, when an affirmative result is obtained at step SP1 by, forexample, applying a static load torque to the corresponding link due tocontact of the link with a wall or an entity, the arithmetic processingunit 90 goes on to step SP2 to control the PWM control unit 91 so as toincrease the compliance of the corresponding motor unit 60 by changingthe duty cycle of the BRAKE_(—)PWM control signal S14 to 0%, and thengoes on to step SP3 to control the PWM control unit 91 so as to reducethe effective value of the current which flows in each coil 68 u, 68 v,68 w of the motor unit 60, thereby obtaining reduced generated torqueT_(out).

Then, the arithmetic processing unit 90 goes on to step SP4 to informthe higher-ranked controller of the execution of the shock absorbingprocess (steps SP2 and SP3), and goes on to step SP5 to wait for aninstruction to stop the shock absorbing process from the higher-rankedcontroller.

When the arithmetic processing unit 90 receives such notification fromthe higher-ranked controller and therefore obtains an affirmative resultat step SP5, it proceeds to step SP6 to change the duty cycle of theBRAKE_(—)PWM control signal S14, that is, the motor viscositycoefficient, to the original value by controlling the PWM control unit91 and thereby return the compliance of the motor unit 60 to the statebefore the shock is detected. Then, the arithmetic processing unit 90proceeds to step SP7 to control the PWM control unit 91 to return theeffective value of the drive current Iu, Iv, Iw to be applied to eachcoil 68 u, 68 v, 68 w of the motor unit 60 to the original value, withthe result that the generated torque T_(out) has a prescribed value.Then, the arithmetic processing unit 90 returns back to step SP1 torepeat the above processes.

As described above, the arithmetic processing unit 90 controls the motorviscosity coefficient and the generated torque Tout of the motor unit 60in the corresponding actuator A₁ to A₂₄ when a static load torque isapplied to a corresponding link.

On the other hand, as is clear from the equation (6), the output torqueτ of the actuator A₁ to A₂₄ is in proportion to the current I_(R1)flowing into the resistance R₁ in the voltage detection unit 82 and thecurrent I_(R1) is in proportion to the fall voltage V_(i) due to thefirst resistance R₁. Therefore, the equation (6) can be transformedinto: $\begin{matrix}{{f(t)} = {{G_{k} \times \overset{.}{\tau} \times \overset{.}{\phi}} = {{G_{k} \times \frac{\mathbb{d}\tau}{\mathbb{d}t} \times \frac{\mathbb{d}\phi}{\mathbb{d}t}} = {K_{i} \times \frac{\mathbb{d}V_{i}}{\mathbb{d}t} \times K_{\theta} \times \frac{\mathbb{d}\phi}{\mathbb{d}t}}}}} & (9)\end{matrix}$where K_(i) and K_(θ) are proportionality constants.

As is clear from the equation (9), the evaluation function f(t) in theequation (6) is derived by a product of a result of multiplying thetemporal variation of voltage V_(i), detected by the voltage detectionunit 82, and the proportionality constant K_(i) and a result ofmultiplying the amount of temporal change of the rotational position φof the rotor shaft 64 of the motor unit 60, detected by the encoder 76,and the proportionality constant K_(θ).

FIG. 26 shows a flowchart of a second load absorbing processingprocedure RT2 which is performed by the arithmetic processing unit 90 ofthe control IC 80. The arithmetic processing unit 90 prevents theactuator A₁ to A₁₄ from being broken in a case where a dynamic loadtorque, that is, the AC component of load, is applied to a correspondinglink, by controlling the PWM control unit 91 following the second loadabsorbing processing procedure RT2, based on a voltage V_(i) and arotational position φ of the rotor shaft 64 of the motor unit 60 so thatthe evaluation function f(t) does not exceed a preset prescribed secondthreshold value SH2, the voltage V_(i) obtained based on the voltagesignal S11 given from the voltage detection unit 82, the rotationalposition φ obtained based on the first and second position signals S10A,S10B given from the encoder 76.

In parallel to the first shock absorbing processing procedure RT1, thearithmetic processing unit 90 starts the second shock absorbingprocessing procedure RT2 from step SP10. At step SP11, the arithmeticprocessing unit 90 calculates the equation (9) based on the voltagesignal S11 and the position signals S10A, S10B and then judges whetherthe result is the second threshold value SH2 or greater. Note that, inthis embodiment, K_(i) and K_(θ) are taken to 1.0 and 4.0, respectively,and the second threshold value SH2 is taken to be a value between 1.3and 4.0 mN−m·rad/S2 which is considered to be most optimal fromexperiment. The arithmetic processing unit 90 repeats this step SP11until an affirmative result is obtained.

When the corresponding link receives a shock and thereby an affirmativeresult is obtained at step SP11, the arithmetic processing unit 90 goeson to step SP12 to change the duty cycle of the BRAKE_(—)PWM controlsignal S14 to 0% to increase the compliance of the motor unit 60 bycontrolling the PWM control unit 91. Then, at next step SP13, thearithmetic processing unit 90 controls the PWM control unit 91 to createthe generated torque T_(out) so that the maximum value of the targetedtorque calculated based on the operation command from the higher-rankedcontroller takes a value between 10 to 20% of the actually calculatedvalue.

Next, the arithmetic processing unit 90 goes on to step SP14 to informthe higher-raked controller of the execution of the shock absorbingprocess (steps SP12 and SP13) and goes on to SP15 where it waits for aninstruction to stop the shock absorbing process from the higher-rankedcontroller.

When the arithmetic processing unit 90 receives such a notification fromthe higher-ranked controller and thereby an affirmative result isobtained at step SP15, it proceeds to step SP16 to return the duty cycleof the BRAKE_(—)PWM control signal S14, that is, the motor viscositycoefficient to the original value by controlling the PWM control unit 91to return the compliance of the motor unit 60 to a state before theshock is detected. Then, at step SP17, the arithmetic processing unit 90returns the effective value of the drive current Iu, Iv, Iw to beapplied to each coil 68 u, 68 v, 68 w of the motor unit 60 to theoriginal value by controlling the PWM control unit 91 in order tothereby return the generated torque T_(out) to the prescribed value.Then, the arithmetic processing unit 90 returns back to step SP11 andrepeats the aforementioned processes.

As described above, the arithmetic processing unit 90 controls the motorviscosity coefficient and the generated torque of the motor unit 60 inthe corresponding actuator in a case where a dynamic load torque isapplied to the corresponding link.

In the aforementioned mechanism, in this robot 1, when a static ordynamic load torque which may break any actuator A₁ to A₂₄ is applied toa corresponding link connected with the output shaft 75 of the actuator,the compliance of the actuator A₁ to A₂₄ increases and the output torqueof the actuator A₁ to A₂₄ decreases, resulting in deformation of thelink according to the load torque.

For example, this robot 1 is capable of previously preventing damage ofthe actuators A₁ to A₂₄ due to shock as in the case where a conventionaltorque limiter 130 is used as described with reference to FIG. 1, and inaddition, the load absorbing is performed only by controlling theelectrical actuators A₁ to A₂₄. As compared with the case of using theconventional torque limiter 130 having a mechanical construction, theactuators A₁ to A₂₄ do not have big differences in performance regardingthe shock absorbing function and also are lightly affected bytemperature.

In addition, the control IC 80 can control the load absorbing operationas well as performing the usual positional control of the actuators A₁to A₂₄. That is, since new units are not required, the actuators A₁ toA₂₄ can be constructed simpler, smaller and lighter, as compared withthe conventional torque limiter 130 having the mechanical construction.

According to the above construction, a sum of absolute values of astatic load torque applied to a link connected to the output shaft 75 ofan actuator A₁ to A₂₄ and the generated torque of the actuator A₁ to A₂₄is obtained as a voltage Vi, and when the sum is the first thresholdvalue SH1 or greater, the actuator A₁ to A₂₄ is controlled so as toreduce the generated torque. In addition, a variation of energy of adynamic load torque applied to the link is detected, and when thedetected variation is the prescribed second threshold value SH2 orgreater, the actuator A₁ to A₂₄ is controlled so as to reduce thegenerated torque. Therefore, as compared with the conventional torquelimiter 130 having the mechanical construction, the actuators A₁ to A₂₄do not have big differences in performance regarding the load absorbingfunction and are lightly affected by temperature, thus making itpossible to realize a easy-to-use load absorbing apparatus.

In addition, by using the load absorbing apparatus according to thisinvention, the actuator A₁ to A₂₄, that is, the entire system can beconstructed simpler, smaller and lighter, as compared with theconventional torque limiter 130 having the mechanical construction.

(5) Load Absorbing Mechanism for the Entire Robot

When overload is applied to a single axis of an actuator motor, theaforementioned load absorbing mechanism for the actuator A₁ to A₂₄ iseffective to appropriately absorb the AC component and the DC componentof the load, so as to prevent breakage of the motor and members such asa link connected to the output shaft of the motor and therefore avoidthe spreading of damage into other members.

On the other hand, multi-joint type robot including leg-type walkingrobot is composed of a plurality of motors for joint actuators and linksconnected with the output shafts of the motors. Therefore, suchsituation may occur that loads in plural motors are totally excessiveeven load in each motor is not excessive. For this situation, a loadabsorbing operation for the entire body is considered to be required,separately from the load absorbing operation for each motor.

Therefore, in the robot 1 of this embodiment, in parallel to executionof absorbing a static and dynamic load torque in each actuator motor, itis monitored whether loads in two or more motors are totally excessive.As a result, it can be detected whether a prescribed part such as armsand legs or the entire body is in an overload state or not, even eachmotor is not in an overload state.

When it is detected that loads in a plurality of motors are totallyexcessive, not the load absorbing operation for each motor but the bodyprotection operation to eliminate the overload state of the entire bodyis performed. The body protection operation here includes cutoff ofpower to relevant motors or all motors of the body and weakening of therelevant motors or all motors of the body. The weakening of a motor isrealized by setting a generated torque to zero or decreasing a viscosityresistance of a motor by gain adjustment.

For example, FIG. 28 shows a situation where the robot 1 is trying tostand up. However, the robot's leg touches an obstacle and if the robot1 keeps on standing up, reactive forces are applied in directions shownby solid line arrows. As a result, loads are applied to the pitch of theright shoulder, the roll and pitch of the right thigh, and the pitch ofthe right ankle. In this case, the loads do not break any axis but maycause plastic deformation in a frame of the body or damage of its cover.

In order to detect such situation, each current for consuming theactuator load in each axis in a joint is detected and all currents aresummed up in a higher-ranked control system and the total current ismonitored as total load in the entire body.

Specifically, a load torque T in each axis of a motor is derived from aproduct of K_(vi) and V_(i) from equation (7), that is, the torque ismonitored based on a voltage applied to the motor. Therefore, the totalT_(sum) of load torques in the entire body is derived as follows:$\begin{matrix}{T_{sum} = {\sum\limits_{k = 1}^{24}T_{k}}} & (10)\end{matrix}$Where k-numbered actuator is A_(k) (note that, 1≦k≦24) and its loadtorque is T_(k).

In a case where the robot 1 stands up as shown in FIG. 28, large load isapplied to the actuators A₅, A₉ of the pitch axes of the right and leftshoulder joints, the actuators A₁₄, A₂₀ of the roll axes of the rightand left hip joints, the actuators A₁₅, A₂₁ of the pitch axes of the hipjoints, and the actuators A₁₇, A₂₃ of the roll axes of the right andleft ankle joints, and the total load torque T_(sum) is derived asfollows: $\begin{matrix}{T_{sum} = {{\sum\limits_{k = 1}^{24}T_{k}} = {( {T_{5} + T_{9} + T_{14} + T_{20} + T_{15} + T_{21} + T_{17} + T_{23}} ) + {O( T_{k} )}}}} & (11)\end{matrix}$In this equation, the total load torque in the other joint actuatorsO(T_(k)) can be ignored.

When the total load torque T_(sum) exceeds a prescribed threshold valuefor a prescribed period of time, the body protection operation such asshutdown is carried out in order to avoid breakage of the body.

FIG. 29 shows a flowchart for detection of a shock load and constantload in each axis of a actuator motor, the detection of overloads inmultiple axes, and the body protection operation in response todetection of the above loads.

At step S1, it is detected from the evaluation function f(t) of equation(6) whether shock load which may cause damage is applied to a jointsingle axis.

When it is detected that shock load has been applied to an actuator, theload absorbing mechanism in the actuator operates in order to avoid theoverload state at step S2. More specifically, the arithmetic processingunit 90 of the control IC 80 performs the load absorbing processingprocedure RT2 shown in FIG. 26, in order to perform the PWM control onthe motor current I_(R1) so that the evaluation function f(t) does notexceed the second threshold value SH2.

Then, the arithmetic processing unit 90 informs the middleware layercontrolling the robot's motion of the execution of the load absorbingprocess at step S3.

When the overload state can be avoided at step S4, a recovery processfor the robot's position and condition is carried out at step S5.

FIG. 30 schematically shows an operation on software for performing theload absorbing operation and recovery operation when shock load isapplied to an actuator.

When it is detected that shock load has been applied to an actuator(S1), the load absorbing mechanism (see FIG. 26) in the actuatoroperates, where the motor current is PWM-controlled to adjust theviscosity coefficient of the motor in order to avoid the overload state(S2). Then, the arithmetic processing unit 90 in the actuator informsthe higher-ranked software, that is, the middleware layer of theoperation of the load absorbing mechanism (S3). When the middlewarelayer determines that the overload state can be avoided (S4), it allowsthe actuator to perform control (S5). The actuator side carries out aposition/condition recovery process and returns the viscositycoefficient of the motor to the original value.

Referring back to FIG. 29, it is determined based on the equation (2)whether a constant load which may cause damage has been applied to ajoint single axis (step S6). When it is determined the constant load hasbeen applied to an actuator, the arithmetic processing unit 90 in theactuator informs the middleware layer (see FIG. 8) controlling therobot's motion of the overload state (step S7).

The middleware layer instructs to decrease a servo gain of the actuatormotor, in response to the notification (step S8).

When the overload state can be avoided (step S4), the position/conditionrecovery process is carried out (step S5).

FIG. 31 schematically shows an operation on software for performing theload absorbing operation and recovery operation when constant load isapplied to an actuator.

When it is detected that constant load has been applied to an actuator(S6), the higher-ranked software, that is, the middleware layer isinformed of this matter (S7). The middleware layer instructs to decreasethe servo gain of the actuator (S8). Then when it is determined that theoverload state can be avoided (S4), it allows the actuator to performcontrol (S5). The actuator side performs the position/condition recoveryprocess to return the servo gain of the motor to the original value.

Referring back to FIG. 29, it is determined based on the above equation(10) or (11) that two or more joints totally come into the overloadstate at the same time due to loads (step S9).

Like the situation shown in FIG. 26, when an overload state is detected,the body protection module is informed of this state via the operationcontrol system of the middleware layer (step S10), to carry out the bodyprotection operation (step Sil).

FIG. 32 schematically shows the structure of the middleware layer todetect an overload state in a plurality of joint axes as a whole andperform the body protection operation. As shown in this figure, the loadmonitoring module and the body protection module are arranged in thismiddleware layer.

The load monitoring module concurrently receives information on a loadstate (for example, a voltage value V_(i) corresponding to a loadtorque) from the actuator motors A₁ to A₂₄, and detect a load state inthe entire body based on the total of the values.

The body protection module carries out the body protection operationwhen the load monitoring module detects the overload state. For example,the body protection module shuts off power of the actuator motors of theall body or relevant parts and weakens the joints.

FIG. 33 schematically shows an operation on software to perform the loadabsorbing operation and the recovery operation when loads in a pluralityof joint axes are totally excessive.

In the middleware layer, the load monitoring module detects an overloadstate in a plurality of joint axes (S9), it informs the body protectionmodule of this state (S11). The body protection module carries out thebody protection operation (S11). For example, it shuts off power to theactuator motors of the all body or to relevant parts and weakens thejoints.

This invention is not limited to a “robot”. That is, this applicationcan be applied to mechanical apparatuses which use electric or magneticfunctions to move like humans, or even to moving apparatuses in otherindustrial fields, like toys.

Further, this invention is applied to a two-legged walking type robot 1in FIG. 1 to FIG. 5. This invention, however, is not limited to this andcan be widely applied to, in addition to robots, other apparatuseshaving servo motors as power sources for moving parts.

Still further, this invention has described a case where a torquemeasuring means for calculating a sum of absolute values of a staticload torque newly applied to a link connected to the output shaft 75 ofan actuator A₁ to A₂₄ and the generated torque of the actuator A₁ to A₂₄and a load absorbing control means for controlling the motor so as toreduce the generated torque when the sum exceeds the prescribed firstthreshold value are realized by the control IC 80 for controlling theposition of the actuator A₁ to A₂₄ based on the operation command COMfrom the higher-ranked controller. This invention, however, is notlimited to this and these means can be provided, separately from thecontrol IC 80.

Still further, this invention has described a case where an energymeasuring means for detecting the variation of energy of a dynamic loadtorque applied to a link connected to the output shaft 75 of an actuatorA₁ to A₂₄ and a load absorbing control means for controlling the motorso as to reduce the generated torque when the variation of energydetected by the energy measuring means exceeds the prescribed secondthreshold value are realized by the control IC 80. However, thisinvention is not limited to this and these means can be provided,separately from the control IC 80.

Still further, this invention has described a case where theproportionality constants K_(i) and K_(θ) in the equation (9) are set to1.0 and 4.0, respectively, and the second threshold value SH2 is set toa value between 1.3 and 4.0 mN−m·rad/S2 which is considered optimal.This invention, however, is not limited to this and theseproportionality constants K_(i), K_(θ) and the second threshold valueSH2 can be set to other values according to the construction of acorresponding actuator.

Still further, this invention has described a case where current I_(R1)flowing in the power line LIN for the drive circuit 81 is detected as avoltage V_(i) and the variations of energy of the static and dynamicload torques applied to a link connected to the output shaft 75 of anactuator A₁ to A₂₄ are detected based on the detected voltage V_(i).This invention, however, is not limited to this and not the currentI_(R1) but another thing can be used, provided that the variations ofenergy of the static and dynamic load torques can be correctly obtainedbased on it.

While there has been described in connection with the preferredembodiments of the invention, it will be obvious to those skilled in theart that various changes and modifications may be aimed, therefore, tocover in the appended claims all such changes and modifications as fallwithin the true spirit and scope of the invention.

1. A robot apparatus having a plurality of movable joint parts,comprising: a plurality of motors for driving the plurality of movablejoint parts; a plurality of first overload detection means for detectingwhether loads in the plurality of motors are excessive, respectively;load absorbing control means for, when any of the plurality of firstoverload detection means detects overload in a corresponding motor,controlling a process to absorb the overload in the corresponding motor;second overload detection means for detecting whether loads in two ormore motors are totally excessive; body protection control means forcarrying out a prescribed body protection operation when the secondoverload detection means detects that the load in two or more motors istotally excessive; and legs configured to move as a two-legged walkingrobot.